---
title: Using Event Triggers to send order-status-change notifications.
description: Succinct, tested, and reusable code recipes for common use cases in Hasura.
sidebar_label: Send a Notification when a Value Changes
keywords:
  - hasura
  - docs
  - recipes
  - event triggers
  - order
  - status change
  - notifications
sidebar_position: 3
---

import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';
import Thumbnail from '@site/src/components/Thumbnail';
import SampleAppBlock from '@site/src/components/SampleAppBlock';

# Send a Notification when a Value Changes

## Introduction

Using Event Triggers allows you to call a webhook with a contextual payload whenever a specific event occurs in your
database. In this recipe, we'll create an Event Trigger that will fire whenever the status of an order changes. We'll
then send a notification to that user.

<SampleAppBlock dependent />

## Prerequisites

Before getting started, ensure that you have the following in place:

- The docs e-commerce sample app deployed to Hasura Cloud.

:::info Tunneling your webhook endpoint from your local machine

If you plan on using a webhook endpoint hosted on your own machine, ensure that you have a tunneling service such as
[ngrok](https://ngrok.com/) set up so that your Cloud Project can communicate with your local machine.

:::

## Our model

Event Triggers are designed to run when specific operations occur on a table, such as insertions, updates, and
deletions. When architecting your own Event Trigger, you need to consider the following:

- Which table's changes will initiate the Event Trigger?
- Which operation(s) on that table will initiate the Event Trigger?
- What should my webhook do with the data it receives?

## Step 1: Create the Event Trigger

Head to the `Events` tab of the Hasura Console and click `Create`:

<Thumbnail
  src="/img/event-triggers/recipes/review-moderation/click-create-event-trigger.png"
  alt="Click Create"
  width="1000px"
/>

## Step 2: Configure the Event Trigger

First, provide a name for your trigger, e.g., `order_status_change`. Choose the `public` schema and the `orders` table.
Then, select the `update` operation under `Trigger Operations`. Then, select the `status` column under
`Listen columns for update`.

Finally, enter a webhook URL that will be called when the event is triggered. This webhook will be responsible for
parsing the body of the request and sending the notification to the user to whom order belongs; it can be hosted
anywhere, and written in any language you like.

The route on our webhook we'll use is `/order-status-change`. Below, we'll see what this looks like with a service like
[ngrok](https://ngrok.com/), but the format will follow this template:

```text
https://<your-webhook-url>/order-status-change
```

:::info Tunneling your webhook endpoint

Since our project is running on Hasura Cloud, and our handler will run on our local machine, we'll use ngrok to expose
the webhook endpoint to the internet. This will allow us to expose a public URL that will forward requests to our local
machine and the server we'll configure below.

You'll need to modify your webhook URL to use the public URL provided by ngrok.

After installing ngrok and
[authenticating](https://ngrok.com/docs/secure-tunnels/ngrok-agent/tunnel-authtokens/#:~:text=Once%20you've%20signed%20up,make%20installing%20the%20authtoken%20simple.),
you can do this by running:

```bash
ngrok http 4000
```

Then, copy the `Forwarding` value for use in our webhook 🎉

:::

Under `Advanced Settings`, we can configure the headers that will be sent with the request. We'll add an
`authentication` header to prevent abuse of the endpoint and ensure that only Hasura can trigger the event. Set the
`Key` as `secret-authorization-string` and the `Value` as `super_secret_string_123`:

<Thumbnail
  src="/img/event-triggers/recipes/review-moderation/event-trigger-secret-header.png"
  alt="Add the secret header"
  width="1000px"
/>

Before exiting, open the `Add Request Options Transform` section and check `POST`. Then, click `Create Event Trigger`.

## Step 3: Create a webhook to handle the request

Whenever a status is changed in the `orders` table, the Event Trigger fires. Hasura will send a request to the webhook
URL you provided. In this example, we're simply going to send a `POST` request. Our webhook will parse the request,
ensure the header is correct, and then send a notification to user in our application.

Event Triggers sent by Hasura to your webhook as a request include a [payload](/event-triggers/payload.mdx) with `event`
data nested inside the `body` object of the request. This `event` object can then be parsed and values extracted from it
to be used in your webhook.

Below, we've written an example of webhook. As we established earlier, this runs on port `4000`. If you're attempting to
run this locally, follow the instructions below. If you're running this in a hosted environment, use this code as a
guide to write your own webhook.

<Tabs
  defaultValue="javascript"
  values={[
    { label: 'JavaScript', value: 'javascript' },
    { label: 'Python', value: 'python' },
  ]}
>
<TabItem value="javascript">

Init a new project with `npm init` and install the following dependencies:

```bash
npm install express body-parser
```

<details>
<summary>
Then, create a new file called <code>index.js</code> and add the following code:
</summary>

```javascript
const express = require('express');
const bodyParser = require('body-parser');

// get express sorted
const app = express();
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: true }));

// notification for user about change in status of order
async function sendNotification(userId, orderId, orderStatus) {
  const response = await fetch(`<YOUR_PROJECT_ENDPOINT>`, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
      'x-hasura-admin-secret': `<YOUR_ADMIN_SECRET>`,
    },
    body: JSON.stringify({
      query: `
                mutation InsertNotification($user_id: uuid!, $message: String!) {
                    insert_notifications_one(object: {user_id: $user_id, message: $message}) {
                        id
                    }
                }
            `,
      variables: {
        user_id: userId,
        message: `Status of Order #${orderId} is now ${orderStatus}.`,
      },
    }),
  });
  console.log(
    `Notification sent. The user has received the following notification: Status of Order #${orderId} is now ${orderStatus}.`
  );
  const { data } = await response.json();
  return data.insert_notifications_one;
}

// now we can write the actual handler using functions
app.post('/order-status-change', async (req, res) => {
  // check the header for auth
  const authHeader = req.headers['secret-authorization-string'];
  if (authHeader !== 'super_secret_string_123') {
    return res.status(401).json({
      message: 'Unauthorized',
    });
  }

  // parse the status from the event payload
  const orderStatus = req.body.event.data.new.status;
  const orderId = req.body.event.data.new.id;
  const userId = req.body.event.data.new.user_id;

  // send notification to user
  await sendNotification(userId, orderId, orderStatus);

  // send some JSON to the client
  res.json({
    orderId: orderId,
    orderStatus: orderStatus,
  });
});

// start the server
app.listen(4000, () => {
  console.log('Server started on port 4000');
});
```

</details>

You can run the server by running `node index.js` in your terminal.

</TabItem>
<TabItem value="python">

Make sure you have the necessary dependencies installed. You can use pip to install them:

```bash
pip install Flask
```

<details>
<summary>
Then, create a new file called <code>index.py</code> and add the following code:
</summary>

```python
from flask import Flask, request, jsonify
import requests

app = Flask(__name__)

# Hasura and OpenAI config
config = {
    'url': '<YOUR_PROJECT_ENDPOINT>',
    'secret': '<YOUR_ADMIN_SECRET>',
}

def sendNotification(user_id, order_id, order_status):
    #REST Api call to HASURA
    response = requests.post(
        config["url"],
        json={
            "query": """
        mutation InsertNotification($user_id: uuid!, $message: String!) {
            insert_notifications_one(object: {user_id: $user_id, message: $message}) {
                id
            }
        }
        """,
            "variables": {
                "user_id": user_id,
                "message": f"Status of Order #{order_id} is now {order_status}.",
            },
        },
        headers={
            "Content-Type": "application/json",
            "x-hasura-admin-secret": config["secret"],
        },
    )
    print(f"Notification sent. The user has received the following notification: Status of Order #{order_id} is now {order_status}.")
    data = response.json()
    return data.get("insert_notifications_one", None)


@app.route('/order-status-change', methods=['POST'])
def order_status_change():
    # Confirm the authentication header is correct
    auth_header = request.headers.get('secret-authorization-string')
    if auth_header != 'super_secret_string_123':
        return jsonify({'message': 'Unauthorized'}), 401

    # Get the user's name and email from the request body
    data = request.get_json()
    order_status = data['event']['data']['new']['status']
    order_id = data['event']['data']['new']['id']
    user_id = data['event']['data']['new']['user_id']

    sendNotification(user_id, order_id, order_status)

    return jsonify({'message': 'Order Change Notification sent!'})

if __name__ == '__main__':
    app.run(port=4000)
```

</details>

You can run the server by running `python index.py` in your terminal.

  </TabItem>
</Tabs>

If you see the message `Webhook server is running on port 4000`, you're good to go!

## Step 4: Test the setup

Head to the `Data` tab and modify one of the orders. Change the `status` to `delivered` and click `Save`. You should see
your server log the following:

```text
Notification sent. The user has received the following notification: Status of Order #<ORDER_ID> is now delivered.
```

We can then head to our `notifications` table in the `Data` tab and we'll also see the new notification 🎉

Feel free to customize the webhook implementation based on your specific requirements. Remember to handle error
scenarios, implement necessary validations, and add appropriate security measures to your webhook endpoint.
